Heretofore, an oil-based ballpoint pen is disclosed, which comprises an oily ink directly filled in the ink container thereof that has a ballpoint pen tip fitted into one end thereof and in which the tip holds a ball at its top end.
The number of the constitutive members thereof is small and the oil-based ballpoint pen of the type is inexpensive. Because of its constitution as above, however, the oil-based ballpoint pen has some problems. Concretely, when the pen is left with its tip downward, then ink may drop out from the gap between the ball and the tip end. In order to prevent such ink dropping, the gap between the two is extremely narrowed and the ink is made to have an extremely high viscosity of from about 10 to 20 Pa·s or so at room temperature. Accordingly, the pen often feels heavy while writing with it, and tends to give thin letters or images.
On the other hand, a water-based ballpoint pen that is filled with a low-viscosity aqueous ink feels light while writing with it, and may give thick letters or images, but it requires a mechanism for ink flow control and is therefore defective in that its structure is complicated and it is expensive.
Further, a shear-thinning ballpoint pen has been disclosed recently, which comprises an oil-based ballpoint pen mechanism structure mentioned above and contains a shear-thinning aqueous ink and an ink follower filled therein (for example, see Patent Reference 1).
Like the above-mentioned oil-based ballpoint pen, the shear-thinning ballpoint pen is inexpensive and its structure is simple, and, in addition, the ink contained in it has a high viscosity while kept statically with no shearing stress applied thereto, and it is stably held in the ballpoint pen mechanism. On the other hand, while in writing with the pen, the viscosity of the ink alone around the ball is lowered owing to the high shear force of the ball that rotates at high speed, and, as a result, the ink may smoothly run through the gap between the ball and the ball-holding area around it owing to the capillary force thereof, and is therefore transferred onto the surface of paper.
Accordingly, when writing with it, the pen may give thick letters and images and may feel lighter than oil-based ballpoint pens. The shear-thinning ink for the pen is controlled to have a shear-thinning index of from 0.1 to 0.6 or so in order that it is prevented from leaking out from the tip end of the pen while in writing with it, and the viscosity of the ink in writing under shear force applied thereto is not so high as compared with that of oily ink. However, when compared with water-based ballpoint pens, the feel in writing with the shear-thinning ballpoint pen is not so good, and, in addition, at the start of writing with it, the ink is often difficult to transfer to the ball.
Moreover, the letters and images written with the shear-thinning ink are difficult to dry, and when they are felt with fingers or rubbed with each other, they may be often transferred onto the non-written background area to stain it.
[Patent Reference 1]
U.S. Pat. No. 4,545,818